Dubro makes these foam wheels that weigh practically nothing and work out really well. They are plastic wheel hubs with foam tires. I have 3 1/2" on my trainer as well as on my other planes. It looks odd, like an Alaska bush aircraft, but what the heck.. what works is what works.

Yeah, do re-check your CG after installing these wheels. It may not matter, but do check it.

CGr.